Flowering Responses of North American Strawberry Cultivars

Karla Patricia Garcia and Chieri KubotaSchool of Plant Sciences, The University of Arizona, Tucson, AZ 85721-0036, USA

APPROACH

REFERENCES

Issues in current classification of cultivars

• US strawberry cultivars are classified based on flowering responses into 3 groups: June-bearing, ever-bearing and day-neutral (Table 1).

Table 1. Strawberry cultivar classification based on flowering response

STRAWBERRY CULTIVAR CLASSIFICATION

June-bearing(short-day)

Typically showing an obligate response, these plants require a photoperiod shorter than a specific threshold (called ‘critical photoperiod’) to initiate flowers. (Darrow and Waldo, 1934; Heide, 2013)

Ever-bearing Generally known to have facultative long-day flowering response, accelerating flowering response when photoperiods are longer (Darrow and Waldo, 1934; Heide, 2013)

Day- neutral Known to flower at the same time irrespective of the photoperiod (Durneret al., 1984; Heide, 2013)

• Current classification is based on less reliable methods of genetic background or /and open field responses.

• There is not a standardized methodology for testing photoperiod effect on flowering in strawberry for accurate cultivar classification.

• Previous research done by Bradford et al. (2010) and Serçe and Hancock (2005) showed that conventional classification of strawberries could have some errors.

SIGNIFICANCE OF PHOTOPERIOD RESEARCH IN STRAWBERRY TOWARDS DEVELOPING GREENHOUSE PRODUCTION PRACTICES

Time-to-flower is critical and must be optimized in fruiting crop production undercontrolled environment. Therefore, finding exact responses to photoperiod in order totrigger flowering is necessary in developing technologies towards off-season greenhousestrawberry production.

Our research objective

To determine photoperiodic response of economically important American strawberry cultivars by identifying day-neutral, facultative vs. obligate responses of cultivars conventionally classified as short-day (SD), ever-bearing (EB), and day-neutral (DN) cultivars using the proper methodology developed in floriculture research.

Eight cultivars were selected from the list of major cultivars planted in US during 2010-2014 and a standard methodology of testing photoperiodic responses widely used in floriculture was applied.

Photoperiodic treatments (Fig 1,2)

RESULTSExperiment 1: Short-day cultivars

Experiment 2: ever-bearing and day-neutral cultivars

APPROACH

Environmental conditions Target average daily temperature: 17 °C

Target light intensity in photoperiodic lighting (Fig.3): 2 µmol m-2 s-1

(400-800 nm)

Lamp source: LEDs containing balanced R:FR light (Phillips, The Netherlands)

Data collection:Microscopic observation of apical meristems and classification of the meristems into 12 developmental stages (From 0-11) (Fig.4)

• SD cultivars: Shoot apical meristem (SAM) dissections week 8.

• EV/DN cultivars: SAM dissection week 7, 8 and 10. Fig. 4. Indices representing strawberry shoot apical meristem stages

(indices: 0-11) used in the present experiments.

• Critical photoperiod for initiating flower development in ‘Chandler’, ‘Radiance’, ‘F-127’, and ‘Shuksan’ were between 13 h and 14 h photoperiod (Fig.5).

• ‘Albion’ showed a relatively weak facultative long-day response.

• ‘San Andreas’ and ‘Monterey’ were found to have a relatively strong facultative long-day response (Fig.6).

• ‘Portola’ showed a consistent day-neutral response in our experiment (Fig.6).

• No difference was found between pre-treatments after treatments in DN/EB cultivars.

• Short-day cultivars receiving 14 h photoperiod developed more runners than plants receiving 11 h -13 h photoperiod (Fig 5).

• Bradford, E., James, F., Hancock, J.F., and Warner, R. (2010). Interactions of temperature and photoperiod determine expression of repeat Flowering in strawberry. Journal of the American Society for Horticultural Science 135, 102-107.• Serçe, S. and Hancock, J. (2005). The temperature and photoperiod regulation of flowering in the strawberry Fragaria chiloensis, F.virginiana and F. x ananassa. Scentia Horticulturae 103, 167-177.• Heide, O.M., Stavang, J.A., and Sønsteby, A. (2013) Physiology and genetics of flowering in cultivated and wild strawberries. J. Hortic. Sci. Biotechnol. 88, 1–18.• Durner, E.F., Barden, J. A., Himelrick, D. G., and Poling E.B. (1984). Photoperiod and temperature effects on flower and runner development in day-neutral, June-bearing and ever-bearing strawberries. Journal of the American Society for

Horticultural Science 110, 808-811.• Darrow, G.M. and Waldo, G.F. (1934). Response of strawberry varieties and species to duration of the daily light period. U.S. Dep. Agric. Tech. Bull. 453.

Fig. 3. ‘Albion’ plants under photoperiodic lighting.

Selected cultivars

Chandler

F-127

Radiance

Shuksan

Selected cultivars

Albion

Monterey

Portola

San Andreas

Experiment 1: SD cultivars Experiment 2: DN/EB cultivars

Cultivars Air temp. in GH (°C)

Air temp. in GC (°C)

Air temp. (24 h) (°C)

Shuksan / Chandler 21.2 ± 0.9 14.8 ± 0.4 17.1 ± 1.1

Radiance / F-127 24.9 ± 1.5 14.1 ± 0.5 17.3 ± 0.9

Cultivars Air temp. in GH (°C)

Air temp. in GC (°C)

Air temp. (24 h) (°C)

DN/EB cultivars 21.3 ± 1.0 15.0 ± 0.4 17.3 ± 0.4

* Mother plants and rooting runners where subjected to pre-treatment conditions (Short-day cultivars to 8 h photoperiod / Day-neutral and ever-bearing cultivars to 8 h and 16 h photoperiod).

Chandler

Chandler F-127 Radiance Shuksan

Fig. 1. Photoperiodic treatments is SD cultivars

Fig 2. Photoperiodic treatments in DN/EB cultivars

Fig. 5. Results of SAM stage (A) of development and runner number (B) in SD cultivars

Fig. 6. Correlation of SAM stage and photoperiod in DN/EV cultivars

A

11h 12h 13h 14h 11h 12h 13h 14h 11h 12h 13h 14h 11h 12h 13h 14h

a ab a b a a a b

B

11h 12h 13h 14h 11h 12h 13h 14h 11h 12h 13h 14h 11h 12h 13h 14h

a a a b a a a b

a a a b a a a b a a a b a a a b

CONCLUSIONS • Short-day cultivars ‘Chandler’, ‘Radiance’, F-127’ and ‘Shuksan’ have a critical photoperiod

between 13 h and 14 h photoperiod at an average daily temperature of 17 °C.• ‘Albion’, ‘Monterey’ and ‘San Andreas’ showed a facultative long-day response and Portola’

showed a day-neutral response at an average daily temperature of 17 °C. • Further experiments must be conducted to confirm cultivar-specific responses and identify

possible interactions in photoperiod response with varied temperatures.